Human monocytes and lymphocytes are activated by a wide variety of antigenic or polyclonal stimulants to produce a multiplicity of immunoregulatory mediators. Activated monocytes produce interleukin-1 (IL-1) which promotes murine thymocyte proliferation but also chenges the thymocyte membrane surface markers from peanut agglutinin positive (PNA plus) to negative and from Lyl plus 2 plus to Lyl plus 2 minus. These phenotypic changes characterize the more mature immunocompetent medullary thymocyte and are associated with the development of IL-1 induced thymocytes to manifest helper activity. IL-1 also enables monocyte depleted T lymphocytes to be activated to produce lymphokines such as IL-2. Sufficient IL-2 activity can be obtained to identify it as a 15,000 MW polypeptide. IL-2 can be differentiated from the 15,000 MW IL-1 using bioassays and they can be separated by chromatography procedures. Helper lymphocytes production of IL-2 is negatively regulated by T suppressor lymphocytes. Furthermore, regulation of immune interferon production correlates directly with that of IL-2 production and therefore also appears to be controlled by suppressor T cells.